Breakthrough infections may not pose major transmission risk, immunologists say

The virus shedding from people with breakthrough COVID-19 infections may be less infectious than that coming from an unvaccinated COVID-19 patient, NPR reported Oct. 12. 

Immunologists said preliminary research has indicated the virus coming from an infected vaccinated person is less infectious because it's coated with antibodies generated from vaccination. While the antibodies don't necessarily prevent infection, they still "should be coating that virus with antibody and therefore helping prevent excessive downstream transmission," Ross Kedl, PhD, an immunologist at the University of Colorado School of Medicine in Aurora, told NPR

The news outlet cited a study of breakthrough infections among healthcare workers in Israel that found for all 37 case patients where data on the source of infection was available, the suspected source was an unvaccinated person with COVID-19. 

While the Pfizer and Moderna mRNA COVID-19 vaccines are injected into the arm muscle and generally expected to generate only the kinds of antibodies that circulate throughout the body, findings recently published in the preprint server medRxiv showed antibodies also showed up in mucosal membranes. 

"This is the first example where we can show that a local mucosal immune response is made, even though the person got the vaccine in an intramuscular delivery," said Jennifer Gommerman, PhD, an immunologist at the University of Toronto who was involved in the research. 

In July, a CDC report made headlines after it found 74 percent of people who contracted COVID-19 after a number of large gatherings in Barnstable County, Mass., were vaccinated. 

However, Dr. Kedl said it's difficult to confirm whether the majority of these breakthrough cases were spread between vaccinated people. 

"In all these cases where you have these big breakthrough infections, there's always unvaccinated people in the room," he told NPR


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